The release notes are indeed very boring. But then again, the only time I ever needed to change my code due to a PHP version was when upgrading to PHP 7, all other upgrades have been basically seamless.
Well written code for PHP 5.4 can work just fine on PHP 8. But a surprisingly large number of projects used outdated functions and dubious syntax that broke in later PHP versions.
Yet there was this case just today where the CMS e107 appears to be broken under PHP 8.3 while working fine under 8.2. So it can happen.